The present invention is concerned with apparatus for measuring differential pressure. A common known form of transducer for this purpose comprises a body with a flexible diaphragm mounted in the body and arranged to be deflected or displaced relative to the body by the differential pressure to be measured applied across the diaphragm. The amount of displacement of the diaphragm is indicative of the pressure difference. Hitherto, a common method of determining the displacement of the diaphragm employs capacitative techniques whereby the diaphragm forms a movable plate of a capacitor. An example of a capacitative differential pressure transducer is described in British Patent Specification No. 1,318,780.
It is also known to use optical techniques for measuring very small displacements. In such known optical techniques, typically a monochromatic light is used to set up interference fringes resulting from reflection from two non-parallel surfaces. The spacing of the interference fringes can be used to determine the angle between the two surfaces and movement of the fringes is indicative of displacement of one surface relative to the other. Such conventional monochromatic interferometry is not capable of giving an absolute measurement of the distance between the two surfaces. The published specification of European Patent Application No. 80100313 (Publication No. 0013974) discloses the use of a Fabrey-Perot interferometer to measure small distances. In the disclosed arrangement, white light is transmitted via optical fibres across a spacing between two accurately parallel partially reflecting surfaces. It is said that only those particular wavelengths of the light that are equal to the half wavelength multiples of the Fabrey-Perot sensor gap widths are transmitted on to a detector arrangement. The detector disperses the transmitted light by means of a prism onto an array of photo detectors so that the wavelengths of peaks in the transmission spectrum can be determined. In this way the spacing between the reflecting surfaces of the interferometer can be calculated uniquely.
The specification of the British Pat. No. 1,168,971 also discloses a Fabrey-Perot interferometer arrangement for measuring the distance between parallel partially reflecting surfaces. In this arrangement the light illuminating the interferometer is monochromatic but its frequency is swept between a first and second frequency. As the frequency is changed, the interference fringe pattern produced in the interferometer changes correspondingly and the distance between the surfaces can be determined by counting the number of changes produced in the fringe pattern at the position of a single photocell.
Neither of the arrangements disclosed in the above mentioned European Publication No. 0013974 and British Patent No. 1,168,971 can be applied to a differential pressure transducer of the moving diaphragm type.